html,
body {
  height: 100%;
  overflow: hidden;
}
* {
  margin: 0;
  padding: 0;
}
.center {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
#root {
  width: 300px;
  height: 300px;
  background: lightblue;
}
.outer {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 200px;
  height: 200px;
  background: lightgreen;
}
.inner {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
  background: lightcoral;
}
